Disclosed is a heat-treated oxidizing agent for gas generating compositions which is obtained by mixing ammonium nitrate with an inorganic compound having as an element one or two or more metal atoms selected from the group consisting of Cu, Fe, Ni, Zn, Co, Mn and Ti and subjecting the mixture to heat treatment, and a gas generating composition containing the same. This invention provides a gas generating composition comprising ammonium nitrate, which can be safely produced, does not change in density with phase transition, and has sufficient combustibility.